1. Field of the Invention
This invention relates to power amplifier systems.
2. Description of the Related Art
RF power amplifiers (PA's) are widely used in the transmitter section of radio transceivers, such as in cellular phones and data cards used with mobile computing devices. The PA provides the last amplification stage for the RF signal being transmitted by the antenna.
In cellular telephone systems, the power delivered by the PA and consequently transmitted by the antenna is typically controlled in a closed loop fashion, where the basestation commands the mobile device to transmit at a proper power level in order to maintain a good communication link to the basestation. As the mobile moves farther away from the basestation, the basestation commands the mobile device to increase its transmit power until a maximum transmit power limit is reached as determined by the mobile device. This maximum transmit power is determined by regulatory standards such as the 3 Gpp TS 25.101. Further limits may be placed on maximum transmit power by safety considerations, based on human body Specific Absorption Rate (SAR) of the RF signal radiated from the antenna of the mobile device. Further, the mobile device may reduce the maximum transmit power if the PA cannot meet transmit adjacent channel leakage limit specifications as required by standards such as 3 Gpp TS 25.101. Such power reduction may be required if challenging peak to average (PAR) in the modulation, impedance mismatch conditions at the antenna (due to the antenna being placed in proximity to a metal surface, for example), temperature changes or other factors cause the PA to operate in a nonlinear mode, resulting in distortion which generates adjacent channel leakage. Finally, it may be prudent to monitor and limit the maximum current drawn from the PA, since for some impedance mismatch angles at the antenna, the PA may draw excessive current, stressing the PA or reducing battery life of the mobile device. Nonetheless, it is still desirable to maximize the mobile's transmit power in order to provide the best possible coverage range and data rates, since often the limiting factor for cellular “dead zones” and low data rates is the mobile device's maximum transmit power capability when at the edge of the cell of mobile phones. An increase in power of just 1 dB can make a difference in coverage area of 14% or more.
FIG. 1 is a block diagram illustrating a conventional RF PA system that monitors and adjusts the PA's transmit power. The PA system includes a transmitter IC (TXIC) 102, a transmit (TX) filter 111, a power amplifier (PA) 104, and a directional coupler 112. PA 104 under supply voltage bias 108 receives RF input signal 106 from TXIC 102, and amplifies it to generate RF output signal 110. The power delivered by the PA 104 at RF output signal 110 is controlled by a TXIC 102 which feeds a composite signal (including at least amplitude modulation information and, phase modulation information and/or frequency modulation information) 106 to the input 107 of the PA 104. TXIC 102 has a variable drive circuit, with the capability to vary the drive level to the input 107 of PA 104, typically by adjusting the gain of an internal drive amplifier 114 and/or adjusting the levels of the RF input signal fed to internal drive amplifier 114. By adjusting the drive level to the input 107 of the PA 104, the RF power level at the output 110 of PA 104 is thereby controlled, and thus the transmitted power is controlled at the antenna (not shown). As mentioned previously, in a typical cellular system (such as WCDMA), the basestation provides commands to each mobile phone to control its transmitted power. A table of values correlating the drive level at TXIC output 106 to the radiated power at the antenna or antenna connector (not shown) is typically maintained by the TXIC 102. The TXIC 102 uses such correlated values to generate the proper drive level at TXIC output 106 in response to such commands received from the basestation. Note that the output 106 of TXIC 102 may feed an interstage TX filter 111 prior to driving PA 107, to reduce spurious noise.
As the mobile device's maximum transmit power capability is reached, the mobile device must limit the transmit power as described earlier. The TXIC 102 may sense the forward coupled power 113 of the RF output signal 110 output from PA 104 using directional coupler 112, and thus estimate the RF power of the RF output signal 115 fed towards the antenna of the mobile device. In this way, a more accurate estimate of transmit power can be made than with the table of values which set drive levels at the TXIC output 106 as mentioned previously. A loop may be formed with TXIC 102 adjusting the drive level at output 106 using its internal drive amplifier 114 based on the forward power 113 sensed at the forward coupled port of direction coupler 112. The conventional RF PA system of FIG. 1 has the advantage of eliminating errors in the transmit power level of the RF signal 115 due to gain variations in PA 104 as well as in TXIC 102.
However, the conventional RF PA system typically cannot accurately limit the maximum transmit power of the RF output signal 115 based on the criteria mentioned earlier. For example, when the PA 104 drives a load (not shown) at an unexpected impedance due to an impedance mismatch at the antenna, increased adjacent channel leakage typically occurs. Under these conditions, the power delivered by PA 104 towards the antenna naturally decreases due to mismatch loss, which helps to mitigate the adjacent channel leakage. However, TXIC 102 will sense forward power 113 at directional coupler 112 which is somewhat unpredicted, due to the directional coupler's lack of 50 Ohms operating environment. At some impedance mismatch angles, the feedback loop including internal drive amplifier 114 tends to adjust the output drive 106 of TXIC 102 to reduce the PA output power 110 excessively. The mobile device's maximum transmit power is thus reduced beyond what is needed, detrimentally affecting mobile phone coverage and data rates. Additionally, the feedback loop operates at a slow rate, since the forward power 113 of the RF signal detected at directional coupler 112 must be heavily averaged to accurately assess the forward power level, causing a large delay in adjustment of the mobile device's maximum transmit power level, during which time the mobile device's maximum transmit power is not optimized. Other factors such as temperature and current drawn by the PA 104 are neither monitored nor estimated, and thus the TXIC 102 may assume the worst case with a tendency towards reducing the PA output power 110 excessively. Finally, the actual power loss caused by various additional components (not shown) between PA 104 and the antenna are not taken into account in adjusting the PA output power 110, and are typically merely estimated, again resulting in suboptimal power control of the PA maximum power.